1. Field of the Invention
This invention relates to a programmable electronic sewing machine which executes a sewing operation on the basis of a predetermined program.
2. Description of the Related Art
Conventional programmable electronic sewing machines comprise sewing control means for controlling various mechanisms including a main shaft on the basis of a predetermined program so that a sewing operation is executed, and speed setting means for setting a rotational speed of the main shaft. In the programmable electronic sewing machines, complicated sewing manners such as pattern sewing can readily be executed automatically or semiautomatically by the sewing control means. Furthermore, the speed setting means includes a speed adjusting dial operated so that the rotational speed of the main shaft is varied. Consequently, a sewing speed can be adjusted according to a required cycle time.
The speed adjusting dial of the speed setting means has a circular scale. However, the speed setting means has no arrangement for displaying the currently set rotational speed of the main shaft. Accordingly, in order that a cycle time may be calculated, the sewing operation needs to be actually executed so that an actual sewing speed is obtained. Thus, the calculation of the cycle time results in difficulty. The prior art has proposed a display provided on an operation panel of the sewing machine so that the main shaft speed set by the speed adjusting dial of the speed setting means is displayed on the display. However, this arrangement results in the following problem. Differing from sewing machines dedicated to the sewing of a straight stitch, the programmable sewing machine is designed to be capable of forming unequal stitches with different stitch pitches according to the entered program. The main shaft can be rotated at high speeds when the stitch pitch is small. However, the main shaft needs to be rotated at low speeds when the stitch pitch is large. The reason for the above-described rotating manner is that a time needs to be gained for horizontally moving a needle relative to a workpiece cloth. In view of this, the main shaft speed is automatically reduced in the programmable electronic sewing machine when the main shaft speed is set at a large value and the stitch pitch is so large that the sewing cannot be executed at the set rotational speed. This does not apply to a sewing machine provided with a needle bar jumping mechanism. However, the sewing machine with such a needle bar jumping mechanism is expensive and is directed to special purposes.
Thus, in the conventional programmable electronic sewing machine, the actual main shaft speed is sometimes rendered lower than a high speed set by the speed setting means depending upon the program. In this case, the cycle time cannot accurately be calculated even when the display is provided for displaying the set speed of the main shaft.